1. Field of the Invention
The present invention relates to a disaster prevention monitoring apparatus which monitors an abnormal state such as a fire, and which remotely controls terminal devices such as zone bells, fire doors and the like when the abnormal state occurs.
2. Description of the Related Art
Examined Japanese Utility Model Application No. Sho. 63-39820 discloses a conventional fault detection circuit for a bell line as shown in FIG. 1, including: a bell B to which a diode is connected in series between first terminal B+ and second terminal B-, and resistor Re and diode De connected in parallel therewith; a first switch Q1 which is connected in series between the first terminal and a positive power source, and which is turned on by a fire signal; a second switch Q2 which is connected in series between the second terminal B- and a negative power source, and which is turned on by the fire signal; a monitor current detection circuit Q3 connected between the first terminal B+ and the negative power source; and a voltage detection circuit Q4 connected between the first terminal B+ and the negative power source.
In the conventional fault detection circuit, during a normal monitor period, a positive voltage is output from the positive power source to the second terminal so that a monitor current flows to turn on the monitor current detection circuit.
When the bell lines are disconnected, the monitor current does not flow so as to turn off the monitor current detection circuit, thereby detecting the disconnection state. When the bell lines are shortcircuited, the voltage detection circuit is turned on.
When a fire breaks out, the first and second switches are on to invert the polarity so that the positive voltage is applied to the first terminal to ring the bell. At this time, the monitor current detection circuit and the voltage detection circuit is turned off.
However, in the conventional fault detection circuit for a bell line, a current is always supplied to lines so as to detect a disconnection or a shortcircuit of the lines. Accordingly, there is a problem that the current for monitoring the lines is consumed even during a normal monitor period.
Furthermore, since two switching devices for inverting the polarity of the terminals are required, the two switching devices must be controlled simultaneously. In other words, there is another problem that, when a plurality of bell lines are connected to such a detection circuit, switching devices for inversion corresponding to the plurality of bell lines are required.